Extraction of deuterium from ammonia synthesis gas

ABSTRACT

In the extraction of deuterium from a mixture of nitrogen and hydrogen commercially prepared for subsequent synthesis to ammonia, the synthesis gas is passed through a first cold region in deuterium exchanging relationship with an exchange liquid, said exchange liquid being selected from the group consisting of ammonia, an amine, a mixture of ammonia and an amine and a mixture of amines, and a stream of hydrogen gas is circulated through a second cold region and a hot region in deuterium exchanging relationship with the exchange liquid. Thus the synthesis gas is not passed through the second cold region or the hot region, and these regions do not therefore have to be capable of accommodating so large a flow as the first cold region.

e E 119 E5112 2 51 Feb. 13, 1973 [54] EXTRACTION OF DEUTERIUM FROMAMMONIA SYNTHESIS GAS [75] Inventor: Howard K. Rae, Deep River, On-

tario, Canada [73] Assignee: Atomic Energy of Canada Limited,

Ottawa, Ontario, Canada [22] Filed: April 3, 1970 [21] Appl. No.: 25,316

. 23/220 [51] Int. Cl. ..C01b 4/00, COlb 4/06 [58] Field of Search..23/2l'0 l, 210, 211, 220

[56] References Cited UNITED STATES PATENTS 3,437,442 4/1969 Poole..23/2l0 l X 3,505,016 4/1970 Walker et al. ..23/210 3,036,891 5/1962Becker ..23/2l0 l X 3,377,135 4/1968 Kenyon 1.

3,464,789 9/ 1969 Courvoisier et al.

3,457,041 7/1969 Klein et al 2,927,003 3/ 1960 Becker 2,787,526 4/1957Sperack ..23/2l0 X Primary ExaminerEdward Stern Attorney-Stevens, Davis,Miller & Mosher [57] ABSTRACT In the extraction of deuterium from amixture of nitrogen and hydrogen commercially prepared for subsequentsynthesis to ammonia, the synthesis gas is passed through a first coldregion in deuterium exchanging relationship with an exchange liquid,said exchange liquid being selected from the group consisting ofammonia, an amine, a mixture of ammonia and an amine and a mixture ofamines, and a stream of hydrogen gas is circulated through a second coldregion and a hot region in deuterium exchanging relationship with theexchange liquid. Thus the synthesis gas is not passed through the secondcold region or the hot region, and these regions do not therefore haveto be capable of accommodating so large a flow as the first cold region.

6 Claims, 2 Drawing Figures 'PATENTED FEB] 3|975 sum 10F 2 FIG.1

INVENTOR HOWARD K. RAE

PATENTEU FEB] 3191a SHEET 2 OF 2 FIG. 2

EXTRACTION OF DEUTERIUM FROM AMMONIA SYNTHESIS GAS This inventionrelates to the extraction of deuterium from a gas stream containinghydrogen, deuterium, and nitrogen.

There is frequently a significant amount of deuteriurn in the gaseousmixture of nitrogen and hydrogen commercially prepared for subsequentsynthesis to ammonia, and various methods have been proposed forextracting some of the deuterium from the synthesis gas before it passesthrough the synthesis plant. Deuterium is usually present in themolecular form HD (hydrogen deuteride). Some of these proposed methodsemploy a bithermal process in which the synthesis gas is passed throughhot and cold regions in countercurrent flow to liquid ammonia. In theseknown methods, the mixture of nitrogen and hydrogen is passed throughthe hot and cold regions. As far as deuteriumextraction is concerned,the nitrogen is superfluous and the volume occupied thereby results inthe deuterium extraction plant having to be undesirably large.

According to this invention, the synthesis gas is passed through a firstcold region in deuterium exchanging relationship with an exchangeliquid, said exchange liquid being selected from the group consisting ofammonia, an amine, a mixture of ammonia and an amine and a mixture ofamines, and a stream of hydrogen gas is circulated through a' secondcold region and a hot region in deuterium exchanging relationship withthe exchange liquid. Thus the synthesis gas is not passed through thesecond cold region or the hot region, and these regions do not thereforehave to be capable of accommodating so large a flow as the first coldregion. 7

To improve the deuterium recovering from the synthesis gas, the firstcold region mentioned in the preceeding paragraph may be split into twocold regions, with the synthesis gas passing through one of these splitregions and the stream of hydrogen gas passing through the other ofthese regions, and the stream of exchange liquid having been split sothat a portion thereof passes through a respective one of these regionsand then combines to form a single stream of exchange liquid thereafter.In this case, the exchange liquid may be other than those mentionedabove, and may for example'be water.

Some nitrogen from'the stream of synthesis gas may be transferred to thestream of exchange liquid, and to reduce the amount of nitrogen in theliquid, means for stripping nitrogen therefrom may be provided. Thisstripping means may include a small purge flow of hydrogen from thehydrogen stream, this purge flow being passed in nitrogen-exchangingrelationship with the exchange liquid after it leaves the cold region inwhich it was in contact with the synthesis gas.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, of which,

FIG. 1 is a flow diagram of a process according to one embodiment, and,

FIG. 2 is a flow diagram of a process according to a second embodiment.

Referring first to FIG. 1, a stream 1 of gas prepared for subsequentsynthesis to ammonia passes up through an upper cold tower C incountercurrent flow to a stream 2 of exchange liquid consisting ofammonia containing potassium amide as catalyst. The synthesis gas stream1 contains mainly hydrogen and nitrogen in the approximate ratio ofthree to one by volume, and also contains a small but significant amountof deuterium. During its passage through the cold tower C the synthesisgas stream 1 loses some of its deuterium to the ammonia stream 2. Thedepleted synthesis gas stream la leaves the top of the cold tower C andis taken to the ammonia synthesis plant.

After leaving the bottom of the cold tower C the enriched ammonia stream2a passes downwardly through a lower cold tower C in countercurrent flowto an upwardly travelling stream 3 of hydrogen gas, which contains somedeuterium. The hydrogen stream 3 loses some of its deuterium to theammonia stream 2a, which thus becomes further enriched..After leavingthe top of cold tower C the depleted hydrogen stream 3a passes upwardlythrough a hot tower H in countercurrent flow to the ammonia stream 217which has just left the bottom of the cold tower C The ammonia stream 2bloses someof its deuterium to the hydrogen stream 30. i

The enriched hydrogen stream 3 leaving the top of the hot tower H isthen passed to the bottom of cold tower C Thus the hydrogen stream 3,30circulates up through the hot tower H, then up through the cold tower Cand back to the hot tower H. After leaving the bottom of the hot towerH, the depleted ammonia stream 2 is passed to the top of the cold towerC Thus the ammonia stream 2, 2a, 2b circulatesdown through the uppercold tower C down through the lower cold tower CL, down through the hottower H and back to the upper cold tower C Both the hydrogen stream 3,3a and the ammonia stream 2, 2a, 2b are in their most deuterium-enrichedcondition between the lower cold tower C and the hot tower H, and thusportions 3c, 2c of one or both these streams can be withdrawn at thispoint for passage through further enrichment stages or for deuteriumextraction treatment. Ammonia and/or hydrogen must of course be suppliedto the respective streams to replace the portions withdrawn. Wherefurther enrichment steps are provided, the replacement ammonia and/orhydrogen may be depleted fluid from a further stage. Makeup flows ofammonia and hydrogen into the respective streams will also be necessaryto compensate for losses in the system.

Thus, the synthesis gas which contains a relatively large volume ofnitrogen, which is of no use in the deuterium-extraction process, isonly passed through the upper cold tower C Only this upper tower C hastherefore to accommodate the large flow of synthesis gas, while thelower cold tower C and the hot tower H have only to accommodate thesmaller circulatory flows of hydrogen and ammonia.

In one specific example of this process, the two cold towers C and Cwere at a temperature of -40 C and the hot tower H was at C. Thesynthesis gas entering the upper cold tower C contained ppm('parts permillion as atom ratio) of deuterium compared to hydrogen in atom ratio,and this was reduced to 76 ppm in the cold tower C giving a recovery of37 percent. The deuterium concentration of the ammonia stream was 380ppm when leaving the hot tower H, 640 ppm when leaving the upper coldtower C and 4,500 ppm when leaving the lower cold tower C The deuteriumconcentration of the hydrogen stream was 120 ppm when leaving the lowercold tower C and 1,300 ppm when leaving the hot tower H.

Instead of using an ammonia stream as the exchange liquid, a suitableamine or a mixture thereof with ammonia or other amine may be used. Theamine is preferably an amine having up to five carbon atoms permolecule, and is preferably a primary aliphatic amine.

FIG. 2 shows a flow diagram which represents an improved processcompared to FIG. 1 in that an improved deuterium recovery can beobtained. As the flow diagram shown in FIG. 2 has some features incommon with that of FIg. l, the same reference numbers will be used toindicate the same features.

In the process of FIG 2, the upper cold tower is divided into two partsC and C 2. The synthesis gas stream 1 passes only through the upper coldtower C l in the upwards direction, and the hydrogen gas stream 3bpasses upwardly through the upper cold tower C 2 after it leaves theupper end of the lower cold tower C After leaving the lower end of thehot tower H, the am monia stream 2, with potassium amide as catalyst, isdivided into two streams 2' and 2". The stream 2' is passed downwardlythrough the upper cold tower C in countercurrent flow to the synthesisgas stream 1, and the stream 2" passes downwardly through the upper coldtower C 2 in countercurrent flow to the hydrogen stream 3. After leavingthe upper cold towers C l and C 2, the ammonia stream 2a and 2a"respectively are combined to form the ammonia stream 2a passingdownwardly through the lower cold tower C With this process, a greaterdeuterium recovering from the synthesis gas is possible, becausedeuterium is transferred from the hydrogen stream 3b to the ammoniastream 2" in the upper cold tower C 2, and this results in the deuteriumcontent of the ammonia stream 2 leaving the bottom of the hot tower H,and stream 2 passing through the upper cold tower C 1, being lower thanthat of the ammonia stream 2 entering the upper cold tower C of theprocess described with reference to FIG. 1. Thus, the ammonia stream 2'is capable of extracting more deuterium from the synthesis gas stream 1in the arrangement of FIG. 2 than the ammonia stream 2 in thearrangement of FIG. 1.

In an example of the process of FIG. 2, the three cold towers C C 2, andC were each at -40 C and the hot tower H was at 70 C. The deuteriumcontent was compared to hydrogen in the synthesis gas stream entering.the cold tower C l was 120 ppm and this stream left the cold tower Cwith a deuterium concentration of 28 ppm, the recovery therefore being77 percent, which is a significant improvement over the processdescribed with referenceto FIG. 1, where the recovery was 37 percent.The deuterium content of the ammonia stream 2 was 110 ppm as it left thehot tower H, the recombined streams 2a and 2a" had a concentration of640 ppm when entering the lower cold tower C and i when leaving C thedeuterium concentration of the stream 2b was 4,500 ppm. The hydrogenstream had a deuterium concentration of 28 ppmas itleft the upper coldtower C 2, 1,300 ppm whenleaving the hot tower H, and 120 ppm whenleaving the lower cold tower C The exchange liquid stream 2 shouldpreferably be divided into the two streams 2 and 2" in the same ratio asthat of the hydrogen flows through the cold towers C l and C 2respectively.

In a similar manner as that described with reference to FIG. 1, theammonia stream 2 may be replaced by a stream of a liquid amine, amixture of ammonia and an amine, or a mixture of two amines, thespecific example being given in the process of FIG. 1 being alsoapplicable to the process of FIG. 2. In the process of FIG. 2, theexchange liquid may be water.

It may be found that, both in the process of FIG. 1 and in the processof FIG. 2, an undesirable amount of nitrogen may be carried along in theammonia or amine stream as it leaves the cold tower C or C l, in whichdeuterium was transferred from the synthesis gas to the ammonia stream.The quantity of nitrogen in the ammonia stream may be reduced to anacceptable amount by means of a nitrogen stripper S (shown only in FIG.2) through which the ammonia stream 20 or 2a passes downwardly whenflowing from the cold tower C or C l to the lower cold tower C A purgingstream 4 of hydrogen is taken from the hydrogen stream 3 as it leavesthe top of the lower cold tower C The purging stream 4 passes upwardlythrough the stripper S in countercurrent flow to the ammonia stream 2aor 20', and then passes upwardly through the upper cold tower C lthereby joining the synthesis gas stream 1.

As the purging stream 4 of hydrogen contacts the ammonia stream 2:: or2a in the stripper S, it takes with it some of the nitrogen in thestream 2a or 2a thereby reducing the amount of nitrogen in the ammoniastream 2a or 2a to an acceptable level. In this way, the nitrogencontent of the exchange liquid stream can be maintained for example inthe range 0.1 to 1 percent of the dissolved gas which is a smallfraction of the normal 25 mole percent in the synthesis gas. To replacethe hydrogen lost from the hydrogen stream, a make-up hydrogen supply 5enters the hydrogen stream 3b between the lower cold tower C and theupper cold tower C,,2.

It may be convenient in some plants to use a small stream of hydrogenfrom some source other than the top of the lower cold tower C to stripnitrogen from the ammonia. Also, other stripping techniques such asflashing a small part of the ammonia after pressure reduction could beused.

A technique for reducing the amount of dissolved nitrogen carried in theliquid stream leaving the bottom of C is to operate that cold exchangetower at a temperature and pressure that favors a lower dissolved gascontent, i.e., at lower temperature and pressure. It is feasible withthe flow arrangement shown in FIG. 2 to operate C l at temperature andpressure conditions much different from C 2. By way of example, C l maybe in the pressure range 300 to 500 psi and at a temperature of C, whileC 2 is at 2,000 to 3,000 psi and 40 C. The amount of mixed nitrogen andhydrogen dissolved in liquid ammonia at the former condition is aboutone-tenth of that at the latter. Thus the size of the stripper S and ofthe purge stream 4 can be reduced considerably and still maintain thelow nitrogen content of the exchange liquid stream 2.

Iclaim:

l. A process for extracting deuterium from a gas stream containinghydrogen, deuterium as hydrogen deuteride and nitrogen including passingsaid gas stream and a first stream of exchange liquid selected from thegroup consisting of ammonia, an amine, a mixture of ammonia and anamine, and mixtures of amines and containing an exchange catalystthrough a first cold region to cause deuterium in said gas stream to betransferred to the exchange liquid, stripping dissolved nitrogen fromsaid first stream of exchange liquid as it leaves said first coldregion, passing a second stream of said exchange liquid containing saidexchange catalyst and a stream of hydrogen gas through a second coldregion to cause deuterium to be transferred from the hydrogen gas to theexchange liquid, combining said first and second streams of saidexchange liquid and passing said combined streams of exchange liquidthrough a third cold region through which said stream of hydrogen gasalso flows to cause deuterium to be transferred from the hydrogen gas tohydrogen gas flowing in turn through the hot region, the third coldregion, the second cold region and then returning to the hot region,dividing said combined streams of exchange liquid after leaving the hotregion into said first and second streams flowing to the first andsecond cold regions respectively, and withdrawing a portion of fluidfrom at least one of the streams between the third cold region and thehot region where said combined exchange liquid streams, passing said"combined streams of exchange liquid and said stream of hydrogen gasthrough a hot regionto cause deuterium to be transferred from saidcombined exchange liquid streams to the hydrogen gas, the stream of theyare enriched with deuterium.

2. A process according to claim 1 including removing dissolved nitrogenfrom said exchange liquid as it leaves said first cold region by passinga purging flow of hydrogen through said exchange liquid.

3. A process according to claim 1 wherein the exchange liquid containsan amine having up to five carbon atoms per molecule.

4. A process according to claim 1 wherein the exchange liquid contains aprimary aliphatic amine.

5. A process according to claim 1 wherein the exchange liquid contains aprimary aliphatic amine having up to five carbon atoms per molecule.

6. A process according to claim 1 wherein the exchange liquid containsammonia.

x v e

1. A process for extracting deuterium from a gas stream containinghydrogen, deuterium as hydrogen deuteride and nitrogen including passingsaid gas stream and a first stream of exchange liquid selected from thegroup consisting of ammonia, an amine, a mixture of ammonia and anamine, and mixtures of amines and containing an exchange catalystthrough a first cold region to cause deuterium in said gas sTream to betransferred to the exchange liquid, stripping dissolved nitrogen fromsaid first stream of exchange liquid as it leaves said first coldregion, passing a second stream of said exchange liquid containing saidexchange catalyst and a stream of hydrogen gas through a second coldregion to cause deuterium to be transferred from the hydrogen gas to theexchange liquid, combining said first and second streams of saidexchange liquid and passing said combined streams of exchange liquidthrough a third cold region through which said stream of hydrogen gasalso flows to cause deuterium to be transferred from the hydrogen gas tosaid combined exchange liquid streams, passing said combined streams ofexchange liquid and said stream of hydrogen gas through a hot region tocause deuterium to be transferred from said combined exchange liquidstreams to the hydrogen gas, the stream of hydrogen gas flowing in turnthrough the hot region, the third cold region, the second cold regionand then returning to the hot region, dividing said combined streams ofexchange liquid after leaving the hot region into said first and secondstreams flowing to the first and second cold regions respectively, andwithdrawing a portion of fluid from at least one of the streams betweenthe third cold region and the hot region where they are enriched withdeuterium.
 2. A process according to claim 1 including removingdissolved nitrogen from said exchange liquid as it leaves said firstcold region by passing a purging flow of hydrogen through said exchangeliquid.
 3. A process according to claim 1 wherein the exchange liquidcontains an amine having up to five carbon atoms per molecule.
 4. Aprocess according to claim 1 wherein the exchange liquid contains aprimary aliphatic amine.
 5. A process according to claim 1 wherein theexchange liquid contains a primary aliphatic amine having up to fivecarbon atoms per molecule.